After months of campaigning the Leave camp has won and Britain will be leaving the E.U. The Post's Adam Taylor talks about what that means for the country and Europe. (Adam Taylor,Jason Aldag/The Washington Post)

With Britain forecast Friday to have taken a historic decision to leave the European Union, E.U. leaders say their ambitions to build a more united continent will be on hold after the bitter campaign exposed deeper troubles within the 28-nation alliance.

Leaders say that a project of greater economic and political integration — first imagined in the ashes of World War II — may have finally hit its limits amid surging skepticism about how much sovereignty to surrender to Brussels.

Even before campaigners for a British exit appeared to prevail in the referendum held Thursday — according to projections from British broadcasters — they had already scored a broader victory.

From cobblestone streets in France to Soviet-era apartment blocks in Slovakia, questions about the European Union’s reach and powers have been given new focus.

Even before the vote, European leaders said that British “leave” campaigners had a sweeping effect in the E.U. capital.

“There is a clear signal all over Europe, not only Britain,” Belgian Prime Minister Charles Michel said shortly before the vote, calling for a large-scale discussion about the role of the E.U., no matter the outcome of the referendum, when the results are known.

“We feel more and more doubts about the European project,” he told the Belgian VRT network.

The first discussions are expected to take place as early as Friday, when leaders take stock of the stunning results. More substantive conversations will begin Tuesday, when E.U. leaders are to meet in Brussels for a summit scheduled long before the British vote.

The British fury about the European Union stems in part from a centuries-old ambivalence about the nation’s relationship with the continent. But much of the anger is shared elsewhere in Europe.

Populist parties from Spain to Poland have exploited the perception that Brussels is home to an unelected bureaucracy, conspiring to suck away self-rule and delivering only slender economic benefits.

On June 23, Britain faces a fateful decision: whether or not to leave the European Union. And the world will be watching. (Daron Taylor,Jason Aldag,Danielle Kunitz/The Washington Post)

“There are cracks everywhere,” gloated the leader of the French anti-E.U. National Front Party, Marine Le Pen, this week.

She said would push for a referendum in her own nation, where 61 percent of residents hold an unfavorable view of the European Union, according to a poll this month from the Pew Research Center.

Her ambitions are likely to be emboldened after the British decision to depart, in what has been called a Brexit.

“It’s more than the future of the United Kingdom in the European Union that’s at stake; it’s the future of the European Union,” French President François Hollande said on the eve of the vote. “After this vote, Europe could equally reach a new step in its construction.”

The European Union’s origins as an effort to keep the peace after the destruction of World War II have long been forgotten, eroding the goodwill that once permeated bargaining in Brussels.

Now, other challenges are far more biting.

“People cannot imagine going to war with each other,” said Fabian Zuleeg, the chief executive of the European Policy Center, a Brussels-based think tank. “The difficulty is that it is taken for granted.”

Critics say the European Union bungled its response to the global economic crisis in 2008 and is still struggling with recovery nearly a decade later. E.U. leaders miscalculated Russia’s anger about their attempts to woo Ukraine, leading to a flat-footed response once Moscow annexed the Crimean Peninsula in 2014.

And the refugee crisis caught European leaders mostly unprepared, despite years in which conflict in the Middle East displaced millions of people.

The result is an alliance where “muddling through” has become the official term for its strategy, and where temporary Band-Aids are applied to a score of challenges because 28 diverse nations cannot agree on a single permanent solution.

Now that Britain appears to have voted to leave, the stresses will become even greater, giving a jolt of energy to anti-E.U. parties elsewhere that will also campaign for referendums of their own.

There will be intense pressure on other political leaders to negotiate special arrangements for their own countries, potentially turning back the clock on European integration. France, Denmark and the Netherlands are among the likely supplicants for fresh freedoms, if not full-out referendums of their own.

Many leaders were saying ahead of the vote that Europe should take a clear-eyed look at its goals — and potentially dial them back.

“We must take a long, hard look on the future of the Union,” European Council President Donald Tusk said this week ahead of the vote. “We would be foolish if we ignored such a warning signal as the U.K. referendum. There are more signals of dissatisfaction with the Union coming from all of Europe, not only from the U.K.”

As the European Union becomes more fractured, it is less likely to be able to respond effectively to the challenges it faces, such as a flat-lined economy in Greece, a resurgent Russia and the constant pressure of refugees fleeing the Syrian conflict.

“It’s like an immune system that is weakened and gets bombarded from the outside,” said Jan Techau, the director of Carnegie Europe, the Brussels office of the U.S. think tank.

One first sign of Europe’s weakened gravitational pull came this week from Turkey, which has engaged in a long, painful negotiation process over E.U. membership to which neither side appears fully committed.

“We can stand up and ask the people, just like the British are doing,” Turkish President Recep Tayyip Erdogan said hours before the British vote, according to Turkey’s state-run Anadolu Agency. “We would ask, ‘Should we continue the negotiations with the European Union or should we end them?’ ”